The present invention relates to a bioreactor for fermenting solids, and a corresponding fermentation method.
The conversion of solid, water-insoluble or particular substances in fermenters involves a wide variety of problems, primarily relating to aeration, mixing and the addition of nutrient media. In addition, if the substrate to be converted is to be synergistically attacked by a number of different microorganisms, it Is required to selectively supply the reaction space with nutrients and oxygen. In large-volume reactors, such fermentations currently cannot be realized due to the complicated mixing and the resulting defective aeration and deficient supply of substrates.
In commercially available reactor systems, the mixing is effected by mechanical agitating systems. In addition, U.S. Pat. No. 4,846,964 describes a fluidized-bed bioreactor system for converting coal to microbiologically liquefied coal products in which an upflowing aqueous stream keeps the coal particles fluidized. The above mixing methods have a drawback in that sufficient mixing and thus a high substrate turnover is no longer possible in the fermentation of higher substrate concentrations or substrates which tend to agglomerate.
Now, it has been the object of the present invention to provide a bioreactor and a fermentation method which overcome the drawbacks of the prior art and, in particular, ensure a sufficient mixing of the substrate to be fermented.
Surprisingly, it has now been found that a sufficient mixing of the reaction medium, which contains solid or water-insoluble fermentation substrates, can be achieved in a bioreactor by purposefully introducing a compressed gas continuously or in compressed gas pulses.
The above object is achieved by a bioreactor having the features as described below, and a method for aerobic fermentation having the features as described below.
The bioreactor serves for the fermentation of water-insoluble or particular substrates, such as wood (which can be degraded only by particular microorganisms due to its lignin content), coals (with the goal of using the liquid fermentation products as starting materials for the chemical industry or for thermal utilization), for the remediation of soils loaded with xenobiotics, for the rapid fermentation of organic waste products, for biological waste water purification, and for the pretreatment of basic materials of the chemical industry.
The proposed bioreactor for the first time permits the optimum aeration and mixing of solid, water-insoluble and particular substances, which is necessary for microorganisms, by using at least one specific nozzle arrangement which is introduced into the substrate to be fermented and pneumatically supplied with compressed gas. Both the supply of oxygen to the microorganisms and the supply thereto of nutrient media, cosubstrates, vitamins, minerals, buffers or antibiotics are effected by a single pneumatic pressure system. In a slightly modified form, the bioreactor can be used In any size from a five-liter laboratory scale to an industrial, multi-hectoliter scale. With simple modifications, the proposed bioreactor can also be used as a conventional liquid/solid phase, solid phase, falling film, fed batch or air-lift reactor.
According to the invention, a first, vertically extending nozzle arrangement can be extended into and retracted from the reaction space of the fermentation vessel. Thus, the nozzle arrangement can be shifted in a vertical direction to enable movement of the nozzle arrangement while pressurized with compressed gas, for a better mixing and aeration. However, if the fermentation process is to proceed under sterile conditions, it is imperative that the reaction space be kept closed. The nozzle arrangement consists of pipes which vertically protrude into the fermentation vessel and are provided with nozzles on their lower ends. In this way, the compressed gas or the liquid bioreactive substrate can be introduced near the bottom of the fermentation vessel. The nozzle arrangement is also suitable for penetrating granular solids present in the fermentation broth. The vertical pipes can have different lengths and can be exchanged. Further, the nozzle arrangement, when retracted from the fermentation vessel, can be cleaned in a simple way.
In addition, a second horizontal nozzle arrangement can be provided which consists of interconnected pipes horizontally extending in parallel through the reaction space. The pipes have nozzles distributed on their coat surface.
The horizontal nozzle arrangement can be used for mixing additionally to the vertical nozzle arrangement.
If the horizontal nozzle arrangement is to rotate for a better mixing of the fermentation substrate, the vertical arrangement of nozzles must be moved upwards, or the vertical pipes must be selected so as not to hinder the rotating of the horizontal nozzle arrangement, i.e., the xe2x80x9cnormalxe2x80x9d pipes must be replaced by shorter pipes.
An advantageous further development of the bioreactor according to the invention has a measuring device in which several measuring electrodes, for example, are provided in a measuring chamber for measuring a medium removed from the fermentation vessel. The measuring chamber is connected with the fermentation vessel through a feed line in order to feed medium to be measured from the fermentation vessel into the measuring chamber. To recirculate the medium into the fermentation vessel after measuring, the measuring chamber is further connected with the fermentation vessel through a recirculating line. The special about this arrangement is the fact that a pressure chamber is inserted upstream from the measuring chamber, where a defined pressure can build which corresponds to the displaced volume in the measuring chamber. According to the invention, the recirculation of the measured medium is effected by applying pressure to the measuring chamber so that the medium is pressed back into the fermentation vessel.
In known bioreactors, measurements are usually performed directly within the fermentation vessel. Since solid substances are contained in the fermentation vessel, the measuring electrodes are often damaged. For external measurement, in known bioreactors, the medium to be measured is transported into a measuring chamber by peristaltic pumps. Such pumps are subject to high wear and are not suitable for transporting larger volumes.
Because the medium is transported by pressure according to the invention, such pumps which are subject to wear and thus to intensive maintenance can be dispensed with. Further, it is possible to transport large volumes. Another advantage of transport by pressure is the fact that the sterility is not affected. This means that no foreign matter gets into the fermentation vessel during the pumping process.
The feeding of medium through the feed line into the measuring chamber can be effected by negative pressure. The measuring chamber according to the invention can further be used as an additional recirculation system. Such a recirculation system can perform an additional aeration of the substrates present in the fermentation vessel.
Further advantageous embodiments of the bioreactor can be seen from the further dependent claims.
In the method according to the invention, for the aerobic fermentation of solid substrates, a reaction medium containing such solid substrates is thoroughly mixed by compressed gas introduced from above into the reaction medium. This thorough mixing dramatically increases the fermentation rate. According to the present invention, xe2x80x9cto introduce from above into the reaction mediumxe2x80x9d means that compressed gas is introduced into the reaction medium by means of suitable devices (such as pipes provided with nozzles) which extend into the reaction medium from above. The length of the pipes can be selected to reach below the surface of the reaction medium at any level in the introduced working condition (i.e., with or without introduced compressed gas).